The invention relates to an interference sonar, that is to say a sonar in which the emission is effected by means of two emitters emitting simultaneously and separated by a length L equal to the maximum length available for the sonar transducer array. By means of interferometry, these emitters generate a multiple beam, the ambiguity of which is eliminated at the reception stage.
An antenna system for an interference sonar of this type has been described in the U.S. Pat. No. 4,234,939 and entitled "Array system having a high resolving power". The emission antenna comprises at least two emission transducers which emit in synchronism and which are placed at the extremities of the reception array. The emission radiation pattern, the amplitude of which varies as a function of the direction .theta. in accordance with F(.theta.): ##EQU1## then exhibits in its angular field a succession of maxima and minima at an interval p such that: ##EQU2##
In these formulae, .theta. is the angle formed by one direction of observation with an axis orthogonal to the line formed by the antennas, c is the velocity of the sound waves in water, f.sub.o is the emission frequency, and .lambda..sub.o =c/f.sub.o is the wavelength of vibration in the radiation medium, i.e. water. The width of the lobes thus obtained by interferometry is approximately one-half of that obtained with an emission base covering the whole of the length L, which improves the resolving power.
It is possible to receive simultaneously in the directions of the emission maxima by predetermining as many reception channels as there are lobes, based on a full reception array which is however composed of a plurality of sources between which a phase displacement is introduced, in such a manner as to cause the maxima of the reception channels to coincide with the emission maxima. In order to cover all the directions of the angular field, provision is made for the creation of further maxima in the emission radiation pattern, coinciding with the minima of the first, and a second series of predetermined reception channels, making use of the same emission and reception sources, either by means of two sequential emissions exhibiting a phase displacement at the same frequency or by means of two simultaneous emissions at different frequencies.
The emissions are acoustic signal pulses of wavelength .lambda..sub.o, which are recurring and which have a duration .tau..sub.e. A frequency band .DELTA.f=1/.tau.e corresponds to this duration. Consequently, corresponding to each frequency included in the band .DELTA.f about the frequency f.sub.o there is a pattern F(.theta.) of the cosine form in accordance with the formula (1) hereinabove, in such a manner that the resulting pattern is not correctly implemented, since there are no well defined maxima or zeros, except for the central maximum.
In fact, the directions of the maxima of the emission pattern which are given by F(.theta.)=1 are such that F.sub.o sin .theta.=2c/L k , where k is an integer. Consequently, when the frequency varies within the frequency band .DELTA.f the direction of the maxima likewise varies in the following manner: ##EQU3## When the frequency of emission F.sub.o undergoes a variation of df, the variation d.theta. corresponding to the direction .theta. has the value of .vertline.d.theta..vertline.=df/f.sub.o .multidot.tan .theta.. In order that the interference pattern should not be degraded to too great an extent, it is necessary to restrict the value of the frequency band, and this demands in the system described hereinabove a minimum duration for the acoustic pulse. It is accepted that d.theta. can reach a maximum of one-fourth of the width of the lobes of the radiation pattern at half power, that is to say at 3 dB attenuation of maximum; this width at half-power is 2.theta..sub.3 .perspectiveto.c/2f.sub.o L=.lambda..sub.o /2L i.e. d.theta..ltoreq..+-..theta..sub.3/2. The variation is a maximum for the limits of the angular field, i.e. -.theta..sub.o and +.theta..sub.o.
For these directions, ##EQU4##
The duration .tau..sub.e of the pulse cannot therefore be less than 1/.DELTA.f.sub.max in order that the pattern should not be degraded to too great an extent.
This restriction presents a twofold difficulty:
In the case of a given angular field, it restricts the resolution in terms of distance, c/2.DELTA.f, since the pulses are too long;
In the case of a given resolution in terms of distance, it restricts the angular field covered.
By way of example, for L=250.lambda..sub.o, a fixed angular field 2.theta..sub.o =30.sup.o, and a frequency of emission F.sub.o =200 KHz, .DELTA.f.sub.max is equal to 750 Hz. Now, in order to obtain a range D=100 meters, i.e. a lateral resolution in terms of distance 2.theta..sub.3 D equal to 0.2 m, a band of 750 Hz is not sufficient. A band at least equal to 3.75 KHz would be required.
The subject of the invention is an interference sonar which does not present the abovementioned difficulties and which more especially is a broad band interference sonar. The particular structure of the system of the sonar according to the invention also permits compressed emission to be achieved.